K2MetadataCompiler doesn't perform explicit check of compilation of
'kotlin' package, thus stdlib build wasn't passing '-Xallow-kotlin-package'.
However, we have special hack for compiling stdlib with contracts: if
'-Xallow-kotlin-package' is passed, then allow compiling contracts for
custom functions. We have to do it this way, because we don't want
stdlib artifacts to be marked with pre-release flag, even though
contracts for custom functions are not enabled yet
(see eae9923dbe).
Therefore, it's crucial to pass -Xallow-kotlin-package properly for that
side-effect.
Dmitry Savvinov
668bd209f2